1. Field of the Invention
The invention relates to a deflection controlled roll including a rotating roll jacket, a rotationally fixed yoke axially passing through the roll jacket, and a plurality of hydrostatic support elements arranged in series on the yoke in the direction of the roll axis, which are each formed by a piston in cylinder unit actuated by pressure fluid to exert a supporting force on the inner side of the roll jacket. The hydrostatic pockets of the hydrostatic support elements are each supplied with pressure fluid via at least one restrictor or capillary. Such a roll is described, for example, in EP-B-0 328 503.
2. Discussion of Background Information
In deflection controlled rolls or deflection compensation rolls, support sources or support elements are used which are charged with oil pressure through a supply line. The respective support source is pressed toward the rotating roll jacket by this oil pressure. Since the piston surface of the support source is smaller than the hydrostatic pocket surface facing the roll jacket, a lower pocket oil pressure is adopted. The pressure difference between the piston pressure and the pocket pressure defines the volume flow which flows via the capillaries disposed between the pocket surface and the piston surface. The respective volume flow is thus adopted at a support source in dependence on the piston pressure.
The support sources are individually charged with an oil pressure for an individual profile correction, i.e. in particular for the correction of certain transverse property profiles of the goods web, in particular of a paper web or of a cardboard web, running through the roll nip. The level of the oil pressures is regulated via an online profile thickness measurement of the goods web.
Large differences can occur between the oil pressures of the different support sources (e.g. from 3.5 to 90 bar from support source to support source) in dependence on the respectively required profile corrections. As already indicated, this results in volume flow differences at the support sources. Friction occurs between the rotating roll jacket and the support sources due to the oil shear in dependence on the jacket speed and to the oil gap level, which is in turn dependent on the volume flow, on the oil temperature and on the pocket pressure. A friction level thus results with a different amount from one support source to the other as a consequence of the large pressure differences and is expressed in temperature differences at the roll jacket. These temperature differences in turn have an effect on the shape of the roll jacket and thus also produce a feedback effect which influences the produced path load profile of the deflection controlled roll.
Since a lower volume flow is adopted with a pressure balance at a support source, a higher temperature results at this support source despite an operationally lower friction level than with higher pressures. However, a higher temperature now results in an expansion of the roll jacket which is expressed in a path load increase in the roll nip. Therefore, the temperature development is expressed in the reverse direction to the desired pressure balance and is thus unwanted. In individual cases, this can even result in instability in the control behavior.
Usually, the temperature development at the support sources is limited by a separate cooling oil flow which is led into the inner space of the roll. For this purpose, a volume flow of lower temperature is distributed in the inner space of the roll via nozzles, the amount of said volume flow being controlled via the return temperature of the roll. Up to now, the same amount of cooling oil is supplied to each support source by such a distribution. However, as a consequence of the previously named volume flow differences, different temperatures are adopted at the support sources despite the supplied cooling oil amount. This state of affairs is documented by the following calculation example:
The present calculation example is a deflection compensation roll of a thickness calender, with the production speed amounting to 1540 m/min. The surface temperature of the roll is, in this case, equal to the return temperature so that no heat flow flows through the jacket.
The technical data relevant to the calculation are as follows:
outer diameter:1016 mminner diameter: 780 mmsupport source size: 70 mm piston diameteroil viscosity:ISO VG 68 (mineral oil)inlet temperature:40° C. for all flows (support source andcooling oil).
The temperature development and the friction level of a support source were examined in the calculation for a minimum (3.5 bar) and a maximum (90 bar) possible piston pressure in dependence on the cooling flow.
FIG. 1 shows a diagram in which the respective oil temperature resulting after a support source is shown over the secondary flow, i.e., the cooling flow, for the minimum and the maximum piston pressure of 3.5 bar and 90 bar respectively. In this connection, the temperature is given in ° C. and the secondary flow in ltr./min. The oil temperature shown was determined directly in the outlet in the direction of jacket rotation behind the support source.
In the inlet of the support source, oil is taken in underneath the support source with the running of the roll jacket at a mixing temperature which results from the injection of the cooling oil into the interior of the roll.
It can be recognized from FIG. 1 that the oil temperatures are much higher for all examined cooling oil flows at a piston pressure of 3.5 bar than at a piston pressure of 90 bar.
The mixing temperature adopted at the interior of the roll approximately corresponds to the local return temperature. FIG. 2 shows a diagram in which the calculated return temperature is entered over the cooling flow (secondary flow) in each case for the two different piston pressures. In this connection, it must be noted that in each case only one support source was examined in the calculation, i.e. a mixing of the oil from a plurality of support sources with different oil pressures and thus different temperatures remains unconsidered.
The adopted local return temperatures show an increasing temperature difference between a support source with a high load and a support source with a low load as the cooling flows become smaller. Such a temperature difference, however, now has a decisive effect on the shape of the rotating roll jacket.